The present invention generally pertains to the nuclear magnetic resonance (NMR) technique of analytical spectroscopy, and particularly to a method of enhancing NMR signals in liquids.
Nuclear magnetic resonance is a well-established spectroscopic method which uses the magnetic interactions of nuclei with their surroundings to provide chemical and structural information about materials. In general, a substance contains many nuclear species (elemental isotopes), each species being present in a certain abundance and each characterized by a magnetogyric ratio .gamma.. In the presence of an applied magnetic field of strength B.sub.o, these nuclei will precess at individual Larmor frequencies: .omega..sub.o =.gamma.(1-.sigma.)B.sub.o, where .sigma. is the chemical shift parameter which describes the strength of the interaction between a given constituent nucleus and the electrons in the host molecule.
Measurements of .omega..sub.o and hence .sigma. are major sources of NMR analytical information. The value of .sigma. for one nuclear species provides information about the chemical structure of the host molecule. Additionally, when different nuclei in the sample are coupled magnetically, the precession frequency is altered, either broadened over a continuous range or split into discrete components. Such structure provides further information about the chemical environment of the nuclear species being observed.
Often it is necessary to obtain NMR spectra of species which have low isotopic abundance and/or low magnetogyric ratio .gamma.. Species having these properties generally have weak signal strength, which necessitates accumulating a large number of successive signals to develop the signal-to-noise ratio to a useable level. Accumulation times for these averages can be as long as several hours and very often can stretch into days. Long accumulation times are detrimental because lengthy analyses occupy an extremely expensive apparatus; furthermore, transient chemical effects may be unobservable if the specimen is chemically unstable over the course of the accumulation.
Many techniques have been developed to solve the problem of long accumulation times. Since the signal-to-noise ratio improves in proportion to the square root of the number of accumulations, the accumulation time is more or less inversely proportional to the square of the strength of the available signal. Therefore a mere doubling of signal strength reduces the required acquisition time by a factor of four. Consequently, much effort has been devoted to increasing the strength of the signals by employing higher magnetic fields (B.sub.o) which increase the spin polarization and by using larger samples containing more observable spins.
Additional signal strength can be obtained when the nuclei under observation are magnetically coupled to a species having a higher .gamma.. Such enhanced polarization is obtained by magnetization transfer from the higher .gamma. species (referred to hereinafter as the I spins) to those with the lower .gamma. (referred to hereinafter as the S spins). The several variations of this general technique rely on the existence of some type of spin-spin coupling between the I and S species for their operation.
In solids, dipolar interaction is the dominant coupling mechanism. Its long range couples all the spins in the sample into sets of thermal reservoirs, each characterized by a particular spin temperature. Polarization transfer here may proceed by thermal equilibration, whereby one establishes contact between hot (low polarization) S and cool (high polarization) I reservoirs. Heat flows from the S to the I spins until equilibrium has been achieved, whereupon the increased polarization of the S species may be observed. The statistical character of the dipolar coupling leads to a monotonic thermal transfer of polarization to the final equilibrium value. Owing to the large dipolar strength, equilibration occurs in times on the order of 20-50 .mu.s for the case of transfer from protons to .sup.13 C nuclei.
In liquids, on the other hand, the anisotropic dipolar interaction is greatly reduced by rapid molecular motion, so that the weaker indirect J coupling becomes dominant. In contrast to the dipolar coupling which operates directly between spins, the J coupling operates by polarization of electrons in chemical bonds. It is characterized by a discrete set of intramolecular spin-spin coupling strengths involving at most a few nuclei, which leads to important differences between the behavior of coupled spins in liquids and solids. The short range of the J coupling leads to oscillations in the polarization transfer accompanied by the development of spin-spin correlations corresponding to mutual precession of the coupled spins. For example in liquids, the polarization oscillates with a period on the order to ten milliseconds for a proton directly bonded to a .sup.13 C nucleus. Hence, polarization strategies based on the thermal models do not apply. Unlike the solid state case, both the timing of the rf pulses used to provide cross-polarization and adjustment of their amplitudes are critical in order to transfer polarization effectively by J coupling.
One method of liquid state signal enhancement is reported by I. Solomon and N. Bloembergen in J. Chem. Phys. 25, 261 (1956). By this technique, the I spins are subjected to steady saturating irradiation, after which the S magnetization is rotated by a .pi./2 pulse into the plane transverse to B.sub.o where its precession is detected by a receiver coil. The steady I irradiation generates thermal processes which increase the S magnetization by a factor between 0 and 1+1/2(.gamma..sub.I /.gamma..sub.S) wherein .gamma..sub.I and .gamma..sub.S are the magnetogyric ratios of the I and S species. A drawback of this technique is that this enhancement factor depends, in detail, on various relaxation processes due to the residual dipolar coupling. The rates of these processes are difficult to predict theoretically and measure experimentally. Further, the rates are often quite slow, thereby necessitating long irradiation times and lengthy delays between successive pulses. Another problem occurs if the magnetogyric ratios have opposite signs, as for example in the important case of .sup.1 H-.sup.15 N systems; for such systems, the polarization transferred to the S species may cancel its initial thermal value, so that one runs the risk of reducing rather than enhancing the signal. Finally, for systems having large .gamma..sub.I /.gamma..sub.S ratios, the maximum enhancement of 1+1/2(.gamma..sub.I /.gamma..sub.S) is substantially less than the enhancement factor .gamma..sub.I /.gamma..sub.S which other techniques can provide.
A method for using both J and dipolar coupling for transferring polarization is disclosed in S. R. Hartmann and E. L. Hahn in Phys. Rev. 128, 2042-53 (1962). The method comprises spin-locking the I spins while simultaneously irradiating the S spins with rf fields having amplitudes which optimally satisfy the condition: .gamma..sub.I B.sub.1I =.gamma..sub.S B.sub.1S. The presence of the S species is detected indirectly by observing the reduction in the I polarization as the I spins transfer their magnetization to the S spins. The chief shortcoming of this technique occurs in the important case of abundant I nuclei and rare S nuclei: the desired S signal must be inferred from the slight drop in the large I polarization. As expected from a method primarily developed for solid state NMR, other problems exist. This method, when applied to liquids, requires extreme care in adjusting the amplitudes of the rf field. A discussion of these difficulties can be found in A. A. Maudsley, L. Muller, and R. R. Ernst, J. Magn Res. 28, 463 (1977). Many other complications peculiar to this technique are not dealt with in Hartmann et al. or in Maudsley et al. because they considered only idealized systems consisting of a single I spin coupled to a single S spin and further assumed irradiation of both species at their Larmor frequencies.
An improvement of the above technique for solids only is disclosed in U.S. Pat. No. 3,792,246 by Gibby et al., filed Nov. 20, 1972. This method proceeds by using the dipolar coupling to transfer heat between various spin systems in solids. The method requires a number of pulse sequences to cool the I spins, to establish thermal contact between the I and S species, thereby cooling the S spins, and to observe the enhanced S polarization generated by the cooling. Unfortunately, this method is not suitable for liquid state NMR since the J coupling in liquids connects only a few spins, unlike the dipolar coupling which connects many. Further, the strength of the spin couplings in liquids is smaller; e.g. a large value of J is 250 Hz for a CH group in a liquid whereas a typical value of dipolar coupling for a CH group in a solid is 50 kHz. The J coupling manifests itself by generating slowly varying periodic spin precessions characterized by well-defined frequencies, whereas the dipolar coupling leads to a rapid monotonic realignment of spin polarization. For this reason, the dipolar thermal equilibration techniques which make the method of Gibby et al. effective in solids cannot be extended to liquids where the weaker J coupling causes oscillatory behavior.
The widespread practical use of cross-polarization techniques for solids compared to liquids indicates the difficulty of enhancing NMR signals in liquids. The weakness and coherent nature of the J coupling generates a type of behavior which has no analog in the dipolar situation. Thus, liquid state methods modeled after dipolar techniques encounter serious obstacles. Use of J coupling requires different approaches if liquid state cross-polarization is to be an effective, practical method.